Inherent versus installed
The inherent characteristic is how flow coefficient varies with travel when the pressure drop across the valve is held constant — a bench property, and what the vendor plots. The installed characteristic is what you actually get in a real system, where opening the valve increases flow, which increases friction losses elsewhere, which steals pressure drop from the valve itself.
That feedback always bends the curve the same way: toward quick opening. The valve gains flow rapidly at first, then saturates as the system swallows the available pressure drop.
Valve authority sets how badly it bends
At N = 1 the valve takes the entire pressure drop and the installed curve equals the inherent one. At N = 0.1 the pipework dominates and even a well-chosen trim produces a curve that is nearly all gain in the first third of travel and nearly flat afterwards.
The three trims
| Trim | Cv relationship | Best suited to |
|---|---|---|
| Linear | Cv ∝ travel | High authority systems, level loops, where ΔP is nearly constant |
| Equal percentage | Cv ∝ R^(travel−1) | Most flow, pressure and temperature loops; authority below ~0.5 |
| Quick opening | Cv ∝ √travel | On-off duty and relief service, not modulating control |
Why equal percentage usually wins
Set the calculator to equal percentage with authority 0.4 and look at the installed curve — it comes out close to a straight line. That is the whole trick: system friction distorts toward quick opening, equal percentage trim distorts the opposite way, and the two roughly cancel.
The payoff is constant gain. A loop whose process gain stays roughly the same from 20% to 80% travel can be tuned once and will behave consistently. A loop whose gain varies five-fold across the range is tuned for one operating point and oscillates or sulks everywhere else — and no amount of PID adjustment fixes a mechanical gain problem.
Field notes
- Authority is decided at sizing time, by how much pressure drop you allocate to the valve. Skimping there to save pump energy buys a control problem that lasts the plant's life — see our valve sizing guide.
- Oversizing compounds the damage. An oversized valve operates at low travel where installed gain is steepest, so resolution and stability are worst exactly where it lives.
- Digital positioners can apply characterisation in software, partially compensating a mismatched trim — useful as a retrofit, but no substitute for correct authority.
- Butterfly and ball valves have inherently different curves and much lower choking limits — check the gas and steam sizing tool for the compressible side.
- Tune after characterising, not before. Tuning constants from the PID tuning calculator assume roughly constant gain; establish that first.
Frequently asked questions
What is valve authority?
The fraction of total system pressure drop taken across the valve at maximum flow. Authority near 1.0 means the valve dominates and its installed behaviour matches its inherent characteristic; low authority means pipe and equipment losses dominate and the installed curve distorts badly.
Why is equal percentage the most common trim?
Because system friction distorts the installed curve toward quick-opening. Equal percentage trim bends the opposite way, so the two effects largely cancel and the installed characteristic comes out close to linear — giving roughly constant loop gain across the range.
What does equal percentage actually mean?
Each equal increment of travel changes flow by an equal percentage of the current flow, not by an equal absolute amount. Mathematically Cv follows R raised to the power of fractional travel minus one, where R is the rangeability, typically 50.
What valve authority should I design for?
Above 0.5 is comfortable and above 0.25 is usually workable with equal percentage trim. Below about 0.2 the valve loses control over most of its travel regardless of trim choice, and the loop becomes difficult to tune.
Idealised model for reference and education; real valves deviate from theoretical characteristics, especially near the seat. See our disclaimer.